The most fundamental reason is not ethnicity, attitude, or even talent -- it's how exciting they are. And by that I mean, how much violence they deliver (since excitement is relative; I, for example, personally find most Mayweather fights exciting.)

You can be a gentleman, like Wlad, or a pantomime villain, like Floyd, but if you don't bring the fireworks people are gonna root against you. And if you keep on winning, eventually they are going to hate you.